In semiconductor manufacturing, etching is required to expose areas of the silicon substrate for diffusion or ion implantation of impurities so as to form integrated circuits in the silicon substrate. One method of etching is by a process of plasma etching or reactive ion etching. In this process, a chemically reactive gas such as CF.sub.4 is used. The surface of the substrate to be etched is covered with a mask, leaving selected areas of the surface exposed. The substrate with a surface to be etched is inserted into a chamber containing the reactive gas. To create the plasma, normally an RF voltage is applied across the gas to cause the gas to disassociate and form various species, such as a positive and negative ions, atoms, such as fluorine, and radicals. The plasma reacts with the surface and forms volatile products, thereby leaving an etched surface in the exposed areas.
By changing the gas and the conditions, as is well known in the art, plasma can be used for depositing a layer on the substrate rather than etching the surface of the substrate.
Plasma etching and deposition rate are dependent on the RF power and frequency (if the power source is RF) gas pressures and compositions, temperature, substrate bias, and gas flow dynamics. Varying any of the above parameters does not permit real time control of the plasma etching or deposition. Yet, such control is desirable. For example, as the plasma etching approaches a critical material interface, it is preferable to slow the etch rate.
Highly reactive species, such as halogen and oxygen, especially in their atomic state, whether neutral or ionized, are desirable in plasma etching. Atomic fluorine in a plasma created from a gas of CF.sub.4 with a small percentage of oxygen is believed to react with silicon and the oxides and nitrides of silicon according to the following reactions: EQU CF.sub.4 .fwdarw.C+4F.sup.O.sbsp.2 CO.sub.2 +4F EQU 4F+Si.fwdarw.SiF.sub.4 EQU 4F+SiO.sub.2 .fwdarw.SiF.sub.4 +O.sub.2 EQU 12F+Si.sub.3 N.sub.4 .fwdarw.3SiF.sub.4 +2N.sub.2
The etching or ashing of organic material, such as photoresist, also is preferably achieved with atomic oxygen. A general reaction would be as follows: EQU CxHy+30.fwdarw.CO.sub.2 +H.sub.2 O
Because of the opacity of air (as well as various glass, quartz, and crystalline window materials) below wavelengths of about 2000 A and the consequent need to operate in a vacuum, this region of the electromagnetic spectrum is commonly called vacuum ultraviolet. The wavelength range corresponds to a photon energy range of 60 KeV to 6 eV. The ionization potentials of most organic compounds, such as CF.sub.4, lie in the energy range 8-13 eV so that organic molecules which absorb vacuum ultraviolet wavelengths will form ion species.